Many water distribution systems throughout the world have been in use for periods approaching or exceeding a century. Over time, the water systems have received varying degrees of maintenance. However, inspection is difficult without costly excavation. Often, no action is taken until a leak is detected, at which time the section surrounding the leak is excavated and repaired. System maintenance has often been limited to monitoring the failure rates for individual lines and performing replacement of an entire line or a long segment of it when leak frequency has exceeded tolerable values. This approach may lead to unnecessary replacement of considerable good pipe. As a result, there exists a need for a cost effective method to ascertain line condition. Since water lines are almost always buried, any applicable inspection method must be capable of operating solely within the bore of the pipe, to detect flaws such as corrosion and cracks through the entire thickness of the pipe.
In order to make inspection cost effective, it must be possible to perform the inspection with minimal preparation of the line, and, in particular, without having to excavate the lines. This means that the inspection device must be capable of accessing the line through existing access points, such as hydrants. The pipes are designed to operate under high internal pressures of, for example, up to 350 PSI. In general, however, the water pressure will be much lower due to operational constraints such as service connections, bell and spigot connections and dead ends. The inspection device must be able to withstand and to operate in such water pressures.
The inspection method must be useable with pipes made of inhomogeneous materials, such as cast iron. In addition, the inspection apparatus must be capable of operating in an environment having the presence of right-angle elbows and tees, large numbers of service taps and fittings, and the relatively large accumulations of scale typical of municipal water systems.
There are several methods of inspection which offer the possibility of measuring pipe condition from the inside, and which are used for this purpose in other applications. Among these are audio inspection, ultrasonic, magnetic flux leakage, eddy current, and remote field eddy current technology.
Ultrasonic methods are used extensively to measure the thickness of many materials with one sided access only, and exhibit very good accuracy in most steels. Unfortunately, they do not work well in cast iron, because the grain size in cast iron approaches the ultrasonic wave length. This results in severe scattering and attenuation of the acoustic signal.
Flux leakage methods are used extensively in oil well casing and petroleum pipeline applications. They are limited by the requirements that the pipe be very clean inside to obtain good flux coupling and to prevent sensor bounce, and that a substantially constant speed be maintained. The scale build-up typical of water lines prevents flux leakage inspection, as does the relatively great wall thickness of these lines. In addition, while this method is effective for the detection of localized sharp edged pits and cracks, it is insensitive to general overall wall loss.
Eddy current methods have been the technique of choice for many years in the inspection of non-magnetic metal piping in applications such as air conditioning units and non-ferrous chemical process piping. These methods use high frequency sinusoidal varying electromagnetic energy and measure the effects of the pipe wall thickness on the field generated by the exciter. In magnetic materials such as cast irons and carbon steels, the depth of penetration of eddy currents is greatly reduced, precluding inspection of the outside of the pipe, particularly when the pipe is of appreciable thickness. Attempts have been made to overcome this limitation by the use of constant magnetic fields to reduce the effective magnetic permeability of the material, but the thickness of typical water lines and the presence of scale make this method impractical for the inspection of these lines. Also, eddy current probes react strongly to changes in the distance between the sensors and the material under inspection, which requires that the inside of the pipe be very clean. For these reasons, this is not a viable method for water line applications.
Remote field eddy current (RFEC) is a relatively new electromagnetic inspection method which has become prominent in the last few years. The term "remote field eddy current" is used to describe the technique in which an alternating magnetic field induced in the pipe by a means such as an exciter or source coil and the field, as modified by the pipe material, is detected at a detector. The detector must be spaced from the exciter coil a sufficient distance to eliminate direct coupling within the pipe between the exciter coil and the detector, and thereby overcome the problems associated with traditional eddy current methods. From classic eddy current equations one can derive an equation illustrating that flux density at any depth will be attenuated and delayed in time (shifted in phase) in a manner related to metal thickness. In particular, eddy current instruments detect a flaw by measuring the reduced attenuation, time delay and field direction the flaw produces as compared with a normal wall thickness. This perturbation in the inner wall electromagnetic field pattern caused by a flaw is highly localized in the vicinity of the flaw and will, to a limited extent, outline the shape of the flaw.